[iOS] History: Fix CSS for showing/hiding Clear Browsing Data button.

Fixes the CSS media queries that hide the button whenever the keyboard
is open.

BUG=300073
R=sergiu@chromium.org

Review URL: https://codereview.chromium.org/25675004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226977 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226977